Typically, back light sources of prior mobile phones include LED and electro-luminescent lamp. The main printed circuit board of the mobile phone mainly includes a first portion corresponding to the keypad module and a second portion corresponding to the LCM module. As known in the prior art, the back light source is used to illuminate the LCM module or the keypad module. It enables users clearly read alpha-numerical keypads in the keypad area or words and patterns in the LCM area.
When using the LED as a back light source, a plurality of the LEDs is welded onto the first portion of the main printed circuit board to illuminate the keypad module. In order to illuminate the LCM module, a holder is designed to include a space for accommodating the LCM module, and a slit is formed at a predetermined location for accommodating a secondary printed circuit board. The secondary printed circuit board carries a plurality of the LEDs to provide a back light source for illuminating the LCM module.
Generally speaking, the secondary printed circuit board is a lengthwise sheet, and it forms a first lateral short leg and a second lateral short leg at its two ends respectively. An electrode is disposed on each lateral leg. In the mean time, a first contact point and a second contact point are disposed on the main printed circuit board corresponding to the aforementioned two lateral short legs. The first contact point and the second contact point provide a first reference power supply and a second reference power supply respectively. After the secondary printed circuit board is disposed within the slit, the first lateral short leg is to be welded onto the first contact point and the second lateral short leg is to be welded onto the second contact point. Therefore, the reference power supplies may provide maximum LEDs.
However, the aforementioned processes of welding the first lateral short leg of the secondary printed circuit board onto the first contact point and welding the second lateral short leg onto the second contact point are somewhat minute and complicated. Accordingly, the cost of these processes is higher.